For measuring a concentration or the like of a particular component in a biosample such as blood or the like in a quick and simple manner, a biosensor using electrochemical means has been put into practice. An example of such a biosensor is a glucose sensor for quantifying glucose in the blood electrochemically.
A glucose sensor includes, as basic elements, an electrode system including a working electrode and a counter electrode, an enzyme and an electron acceptor. The enzyme selectively oxidizes glucose in the blood to generate gluconic acid, and also reduces the electron acceptor to generate a reductant. By applying a certain level of voltage to the reductant by the electrode system, the reductant is re-oxidized. At the time of the re-oxidation, an electric current is generated. Since the value of the electric current depends on the concentration of glucose in the blood, the glucose in the blood can be quantified.
According to a conventional biosensor, silver paste is screen-printed to form a lead wire, and conductive carbon paste is printed on the lead wire to form an electrode system (Japanese Laid-Open Patent Publication No. H8-15220 and Japanese Laid-Open Patent Publication No. H8-5600). According to such a conventional biosensor, the electrode system is formed of conductive carbon. Therefore, the resistance of the wire is large, and thus the potential of a surface of the electrode is easily dispersed. This decreases the measuring precision of the glucose concentration. In addition, a surface of the silver paste exposed to air is easily oxidized into silver oxide, which has a high resistance. Therefore, the resistance of the lead wire is not stable.
International Publication WO2010/004690 pamphlet proposes an electrochemical sensor including a conductive layer provided on an insulating base material, a first carbon layer provided on the conductive layer, and a second carbon layer for covering the first carbon layer. The electrochemical sensor described in Patent Document 3 is proposed for the purpose of performing a measurement for a long time and in repetition. This indispensably causes constrains such that, for example, noble metal needs to be used as the material of the conductive layer, especially as the material of a reference electrode, and the conductive layer needs to be formed by vacuum film formation. Production of such an electrochemical sensor has problems of increasing the cost of the material and also increasing the number of production steps. In addition, in order to increase the adhesiveness between the insulating base material and the conductive layer, the conductive layer needs to be formed of Cr, Ti or the like, which is highly adhesive. This further increases the number of production steps and the cost.